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SnowFlurry 6 2020/21 | Mechanical conversion

How does snow become drift snow?

by Stefanie Höpperger 12/26/2020
In addition to the constructive and degradative transformation, which have often been discussed in SnowFlurry, there is also mechanical transformation. Today we ask ourselves: What happens to snow crystals during mechanical transformation? What does the formation of drift snow have to do with it and how does it actually develop?

First of all, I would like to briefly explain the difference between degradative and mechanical transformation. In both cases, the crystals become smaller, move closer together and a bound layer of snow is formed. However, the process leading up to this point is different: both the degradative and the constructive transformation take place within the snow cover. So we are talking about the crystals that are already on the ground. The transformation processes depend on temperature, water vapor, pressure, etc. You can find a detailed description of degradative and constructive metamorphosis in SnowFlurry No. 6 2019/20. Mechanical transformation, on the other hand, is caused by external influences, especially wind. It is a metamorphosis process like the other two, but rather a "destruction" of the snow crystals.

If there is no wind at all, the snow crystals fall from the sky in their original, dendritic shape (hexagonal) and only begin to transform once they have reached the ground. If, on the other hand, it snows when there is wind, the snow crystals are already whirled around in the air by the wind. As a result, they collide with each other, branch out and are torn apart again. Small branches break off. This can also happen if they collide with a hard obstacle. As the snow crystal whirls around, this process is repeated several times, causing the crystal to be destroyed further and further and to take on an ever smaller and rounder shape. Finally, the crystals come to rest on the ground or snow surface as a mixture of round grains and small and larger pins (felt).

The now small and roundish shape of the crystals allows them to move close together and therefore bond well with each other. The result is a bound layer of snow that reacts strongly to stress and acts very well as the "board" of an avalanche.

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The mechanical transformation and formation of drifting snow does not only take place in the air when it is snowing. An already existing, soft snow surface can also be transported by the wind. While this is happening, the crystals are destroyed in a similar way by the mechanical transformation. The crystals are lifted from the snow surface by the wind or thrown into the air. There they bounce against each other several times, destroying them and causing them to break. At some point they come to rest on the leeward side, as well as in gullies and hollows. Fresh drift snow is formed again.

The difference is that the crystals on the snow surface may have already undergone several forms of metamorphosis. The crystals therefore not only have the shape of a hexagonal snow star, as with fresh snow. They can also consist of angular crystals, felty snow (pins, branches), etc. The following applies here: the softer the snow surface, the easier it is carried away by the wind.

In other words, drift snow is a product of mechanical transformation and is shaped by external influences, especially the wind. Drift snow consists mainly of round grains as well as branches and sticks. The result is always a bound layer of snow, which provides an important ingredient for an avalanche - the "board". However, a weak layer is also required. This is almost always present in the formation of drift snow and consists, for example, of loose new snow crystals that have fallen without the influence of wind. Surface frost, angular crystals or a layer in the old snow cover can of course also act as a weak layer.

For example: Let's assume we have a compact and stable old snow cover. It starts to snow without wind. Beautiful, dendritic snow crystals form a new layer on the snow surface. Over time, the wind picks up more and more and blows at a medium speed above the load-bearing capacity (moderate wind 20-28km/h; fresh wind 29-38km/h). The mechanical transformation now takes place on the one hand in the still falling fresh snow and on the other hand in the loose snow crystals on the ground, which previously fell without wind and are now whirled up. The result is usually a weak layer of large, loose fresh snow crystals that are poorly connected to each other, covered by a bound layer of drift or fresh snow. This means that there are already two ingredients for an avalanche: the weak layer overlaid by a board (bound snow layer). If the slope gradient is steep enough and a break in the weak layer can be initiated, an avalanche is very likely.

Properties and interesting facts about drifting snow:

  • As already mentioned, drifting snow consists of a bound layer of snow that is easily disturbed and can transfer tension well. However, bound snow does not mean that the snow layer is compact! It can certainly feel like powder fun, because powder snow can also be bound!

  • Furthermore, the colder and drier the snow, the more brittle it is. The material can then be seen as a solid body that can be broken. The breaking of the crystals can even be heard as crunching and cracking. The more brittle the crystals are, the easier they break and the more sensitive they are to stress, for example from winter sports enthusiasts.

  • When the snow temperature is close to the melting point (0°), the snow becomes more plastic. A drifting snow problem then becomes less relevant. The snow can be "stretched", deformed and becomes pliable without the crystals breaking. This effect is easy to observe when the snow hangs curved down from a roof, for example, and simply won't break.

  • The hazard pattern 6 - loose snow and wind (drift snow) is usually only present for a rather short time. The duration depends on temperature and settlement. With one exception: when a snow surface of loose, angular crystals - a result of the build-up transformation - is transported. In this case, hard and brittle drift snow layers are formed, which can be disturbed over a longer period of time.

  • Drift snow can already be formed at a wind speed of 15km/h. The snow load increases sharply as the wind speed increases.

  • Floating snow can be easily recognized in comparison to other hazard patterns in the terrain by wind signs such as wind gusts, dunes, flags, anraum, etc., unless it is overlaid by fresh snow.

The Schneestöberer wish you a Merry Christmas and a Happy New Year!

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